1. Field of the Invention
The present invention relates to a multipoint focus detecting apparatus which can determine a focus state at each of a plurality of focus detection zones, and which is suitable for an optical instrument such as an SLR camera.
2. Description of the Prior Art
In recent years, SLR cameras provided with a multipoint focus detecting unit for determining a focus state (defocus) at each of a plurality of focus detection zones (focusing points) have been developed. A typical conventional exit-pupil-dividing multipoint focus detecting unit is provided with a plurality of exit-pupil dividing devices, a plurality of pairs of light distribution forming devices and a plurality of array type photoelectric conversion sensors (e.g., CCD line sensors). Each exit-pupil dividing device functions to divide the exit pupil of a photographing lens into two detection sub-zones. Each exit-pupil dividing device is composed of, e.g., a separator mask having a pair of apertures. Each pair of light distribution forming devices receive light bundles which are passed through the corresponding aforementioned two detection sub-zones to form a corresponding pair of light distributions, respectively, whose relative positions vary in accordance with the variation in position of the focal point of the photographing lens. Each pair of light distribution forming devices are composed of, e.g., a pair of separator lenses. The plurality of array type photoelectric conversion sensors are arranged in a direction orthogonal to the direction in which the exit pupil of the photographing lens is divided by each of the aforementioned exit-pupil dividing devices. The conventional exit-pupil-dividing multipoint focus detecting unit is further provided with an opaque field mask (focus detection zone determining plate) having a plurality of apertures which determine the plurality of focus detection zones, respectively. The field mask is positioned in a plane located at a position optically equivalent with a film surface (i.e., a focal plane of the photographing lens). Each of the plurality of apertures of the field mask determines the size of the bundle of rays which are incident on the corresponding separator mask (the exit-pupil dividing device). However, the size of each array type photoelectric conversion sensor is generally smaller than that of the corresponding focus detection zone, while the space between two adjacent array type photoelectric conversion sensors is generally narrower than that between the corresponding two adjacent focus detection zones. For this reason, the conventional exit-pupil-dividing multipoint focus detecting unit is further provided between the field mask and the exit-pupil dividing devices with at least one condenser lens (sometimes with at least one prism) for converging each of the object light bundles which are respectively passed through the focus detection zones (i.e., the apertures) of the field mask and for deflecting each object light bundle in a direction of passing through the corresponding separator mask (the exit-pupil dividing device) so that the light distribution of the object light bundle (i.e., a object image) is formed on the corresponding array type photoelectric conversion sensor via the corresponding pair of separator lenses (the corresponding pair of light distribution forming devices).
FIGS. 5A, 5B and 5C each show a diagram of fundamental elements of a focus detection optical system of a conventional multipoint focus detecting apparatus. In each of these drawings, reference numerals 101, 102 and 103 represent the exit pupil of a photographing lens, the entrance pupil of the focus detection optical system and the aforementioned field mask, respectively.
In the focus detection optical system of a conventional multipoint focus detecting apparatus shown in FIG. 5A, two object light bundles which are passed through two focus detection zones (i.e., two apertures) of the field mask 103 are converged and deflected by a rotationally-symmetrical condenser lens 111 toward two separator masks (the aforementioned exit-pupil dividing devices) 113 and 114, respectively, so that the light distribution of each object light bundle (i.e., a object image) is formed on the corresponding line sensor (the aforementioned array type photoelectric conversion sensor) 117 or 118 via the corresponding pair of separator lenses (the corresponding pair of light distribution forming devices) 115 or 116. In this conventional multipoint focus detecting apparatus shown in FIG. 5A, more than one focus detection zone of the field mask 103 are covered by a single condenser lens, i.e., the rotationally-symmetrical condenser lens 111. Since the condenser lens 111 has the same power in both horizontal and vertical directions and is rotationally symmetrical about the optical axis thereof, neither the light distribution of the object light bundle formed on each of the two line sensors 117 and 118 nor the distortion thereof can be corrected by the condenser lens 111, even though the exit pupil 101 and the entrance pupil 102 are successfully made to coincide with each other in both horizontal and vertical directions by the condenser lens 111. Therefore, neither the area of each focus detection zone of the field mask 103 nor the space between the two focus detection zones of the field mask 103 can be made large. Furthermore, if it is attempted to make the exit pupil 101 and the entrance pupil 102 coincide with each other, the degree of freedom in arrangement of the separator masks 113 and 114 is somewhat restricted.
In the focus detection optical system of a conventional multipoint focus detecting apparatus shown in FIG. 5B, a condenser lens is disposed for each focus detection zone, namely, two condenser lenses 121 and 122 are disposed for the two focus detection zones of the field mask 103, respectively. In the focus detection optical system of this conventional multipoint focus detecting apparatus, two object light bundles which are passed through the two focus detection zones of the field mask 103 are converged and deflected by two condenser lenses 121 and 122 to two separator masks (the aforementioned exit-pupil dividing devices) 123 and 124, respectively, so that the light distribution of each object light bundle (i.e., a object image) is formed on the corresponding line sensor (the aforementioned array type photoelectric conversion sensor) 127 or 128 via the corresponding pair of separator lenses (the corresponding pair of light distribution forming devices) 125 or 126. In this conventional multipoint focus detecting apparatus shown in FIG. 5B, since a condenser lens (121 or 122) is disposed for each of the two focus detection zones of the field mask 103, the exit pupil 101 and the entrance pupil 102 can be made to coincide with each other in both horizontal and vertical directions by the condenser lens 121 and 122, and at the same time, the light distribution of the object light bundle formed on each of the two line sensors 127 and 128 and the distortion thereof can be corrected by the condenser lenses 121 and 122. However, in this conventional multipoint focus detecting apparatus, more than one condenser lens is required, which extensively increases the number of lens surfaces that are to be formed on molded condenser lenses.
In the focus detection optical system of a conventional multipoint focus detecting apparatus shown in FIG. 5C, a condenser lens is disposed for each focus detection zone, namely, two condenser lenses 131 and 132 are disposed for the two focus detection zones of the field mask 103, respectively. Furthermore, a prism is disposed for each condenser lens; namely, two prisms 133 and 134 are disposed for the two condenser lenses 131 and 132, respectively. Each of the two prisms 133 and 134 deflects a bundle of rays which are passed through the corresponding condenser lens 131 or 132 so that the two light bundles, which are respectively passed through the two condenser lenses 131 and 132, approach each other. According to this optical structure, the space between the two pairs of object images which are respectively formed on two line sensors 139 and 140 via two separator masks 135 and 136 and two pairs of separator lenses 137 and 138 becomes small, which makes it possible to arrange the two line sensors 139 and 140 to be close to each other. However, in this conventional multipoint focus detecting apparatus, a condenser lens and a prism are required for each focus detection zone, so that the number of elements of the focus detection optical system increases. This makes it difficult to assemble the multipoint focus detecting apparatus.
In order to reduce detection error, the condenser lens (or lenses) used as an element (or elements) of the focus detection optical system of an exit-pupil-dividing multipoint focus detecting apparatus should satisfy the following three conditions (1) through (3), and it is preferable for the overall focus detection optical system to satisfy the following three conditions (4) through (6):
(1) The exit pupil of the photographing lens and the exit-pupil dividing devices (separator masks) are optically conjugate to each other in the direction in which the exit pupil of the photographing lens is divided by each of the exit-pupil dividing devices.
(2) The exit pupil of the photographing lens and the exit-pupil dividing devices (separator masks) are optically conjugate to each other in a direction orthogonal to the direction in which the exit pupil of the photographing lens is divided by each of the exit-pupil dividing devices.
(3) The light distribution of the object light bundle formed on each line sensor and the distortions thereof are adequately corrected.
(4) The degree of freedom in arrangement and structure of each element of the focus detection optical system is high.
(5) The number of elements of the focus detection optical system is as small as possible.
(6) The plurality of focus detection zones can be arranged in a wide range and/or the area of each focus detection zone is large.
However, in the focus detection optical system of a conventional exit-pupil dividing multipoint focus detecting apparatus, although conditions (1) through (3) are moderately satisfied by the condenser lens (or lenses), conditions (4) through (6) are not satisfied. Specifically, there are no conventional exit-pupil-dividing multipoint focus detecting apparatuses which can satisfy condition (6).
The present invention has been developed in view of the aforementioned problems in conventional multipoint focus detecting apparatuses, and an object of the present invention is to provide a multipoint focus detecting apparatus which makes it possible for a plurality of focus detection zones to be set in a wide range and/or that the area of each focus detection zone to be made large, without increasing the number of elements of the focus detection optical system and without deteriorating the precision in focus detection.
To achieve the object mentioned above, according to an aspect of the present invention, a multipoint focus detecting apparatus of a camera is provided, including a plurality of exit-pupil dividing devices for dividing an exit pupil of a photographing lens into a plurality of detection sub-zones in a first direction; a plurality of pairs of light distribution forming devices, each pair receiving light bundles which are passed through a corresponding pair of the plurality of detection sub-zones to form a corresponding pair of light distributions, respectively, relative positions of which vary in accordance with a variation in position of a focal point of the photographing lens; a plurality of array type photoelectric conversion sensors arranged in a second direction orthogonal to the first direction in which the exit pupil is divided by each of the plurality of exit-pupil dividing devices; a focus detection zone determining device that is positioned in a plane located substantially at a position optically equivalent with a focal plane of the photographing lens to determine a plurality of focus detection zones; and at least one condenser lens positioned between the focus detection zone determining device and the plurality of exit-pupil dividing devices so that light bundles which are respectively passed through the plurality of focus detection zones pass through the at least one condenser lens. An exit surface of the at least one condenser lens is formed as a curve in cross section taken along a plane extending in both a direction of an optical axis of the photographing lens and the first direction, and is further formed as a curve having a varying curvature in cross section taken along a plane extending in both the direction of the optical axis of the photographing lens and the second direction, so that the exit pupil and the plurality of exit-pupil dividing devices become optically conjugate to each other in the first direction.
According to this arrangement, the exit pupil of the photographing lens and the exit-pupil dividing devices become optically conjugate to each other, and at the same time, the light distribution of the object light bundle formed on each line sensor and the distortion thereof are moderately corrected. This makes it possible for the plurality of focus detection zones to be set in a wide range and/or that the area of each focus detection zone to be made large with a high degree of precision in focus detection. Further, since the plurality of bundle of rays which are respectively passed through the plurality of focus detection zones are converged and deflected by a single condenser lens, it is easy to make and assemble the condenser lens, while the degree of freedom in arrangement of the condenser lens is high.
Preferably, the exit surface is rotationally-asymmetrical about the optical axis thereof.
Preferably, at least one condenser lens is a single condenser lens, so that the light bundles which are respectively passed through the plurality of focus detection zones pass through the single condenser lens.
Preferably, each of the plurality of exit-pupil dividing devices is a separator mask having at least one aperture.
Preferably, each pair of the plurality of pairs of light distribution forming devices is a pair of separator lenses.
Preferably, each of the plurality of array type photoelectric conversion sensors is a CCD line sensor.
Preferably, the focus detection zone determining device is a field mask having a plurality of apertures which determine the plurality of focus detection zones, respectively.
Preferably, the multipoint focus detecting apparatus is incorporated in an SLR camera.
According to another aspect of the present invention, a multipoint focus detecting apparatus of an AF camera, includes a first opaque plate that is positioned in a plane located substantially at a position optically equivalent with a focal plane of a photographing lens of the AF camera, the opaque plate including a plurality of apertures respectively determining a plurality of focus detection zones; at least one second opaque plate for dividing an exit pupil of a photographing lens into a plurality of detection sub-zones, each of the at least one second opaque plate having at least two apertures; a plurality of pairs of separator lenses, each of the pairs of separator lenses receiving light bundles which are passed through a corresponding pair of the plurality of detection sub-zones to form a corresponding pair of light distributions, respectively, relative positions of which vary in accordance with a variation in position of a focal point of the photographing lens; a plurality of line sensors arranged in a second direction orthogonal to a first direction in which the exit pupil is divided by each of the at least one second opaque plate; and a condenser lens positioned between the first opaque plate and the at least one second opaque plate so that light bundles which are respectively passed through the plurality of focus detection zones pass through the condenser lens. An exit surface of the condenser lens is formed as a curve in cross section taken along a plane extending in both a direction of an optical axis of the photographing lens and the first direction, and is further formed as a curve having a varying curvature in cross section taken along a plane extending in both the direction of the optical axis of the photographing lens and the second direction, so that the exit pupil and the at least one second opaque plate become optically conjugate to each other in the first direction.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 11-302560 (filed on Oct. 25, 1999) which is expressly incorporated herein by reference in its entirety.